1. Field of the Invention
The present invention relates to a method of producing a magnesium fluoride coating, an antireflection coating having a good antireflection effect and a low refractive index, and an optical element including the antireflection coating.
2. Description of the Related Art
Conventional antireflection coatings for optical elements, such as lenses, required to have an antireflection effect are known to have a multilayer structure formed by alternately depositing a high-refractive index substance such as titanium oxide (TiO2) or zinc oxide (ZnO2) and a low-refractive index substance such as magnesium fluoride (MgF2) by a vacuum vapor deposition process. In particular, layers located on the air side are preferably made of a low-refractive index substance; hence, MgF2, which has a refractive index nd of 1.38 for light with a wavelength of 587 nm, has been used to form such layers.
Japanese Patent Laid-Open No. 59-213643 discloses a method of producing an MgF2 thin film by a wet process instead of vacuum vapor deposition.
Japanese Patent Laid-Open No. 59-213643 also discloses a method of producing magnesium fluoride by a thermal disproportional reaction. In particular, a fluorine-containing organic magnesium compound or a precursor thereof is applied to a substrate and is then disproportionated by heating, whereby magnesium fluoride is produced.
In this method disclosed in Japanese Patent Laid-Open No. 59-213643, the temperature required to form a magnesium fluoride film by the thermal decomposition or disproportion of the fluorine-containing organic magnesium compound is 300° C. or higher.
When the fluorine-containing organic magnesium compound is magnesium fluorocarboxylate, no MgF2 film can be obtained unless magnesium fluorocarboxylate is heated to a higher temperature in accordance with the increase in molecular weight of a fluorocarboxylic acid used.
In the case of preparing an optical element by forming an MgF2 thin film on a base, the heating of the base to a high temperature for the purpose of forming the MgF2 thin film possibly causes a reduction in dimensional accuracy and will cause significant damage depending on a material used to form the base.